A range sensor using a conventional position detecting element is, for example, disclosed in JP62-28610. FIG. 7 shows the optical system that is the basic component of this range sensor.
FIG. 7 includes a light source 101 using an emitting element such as a light-emitting diode, a semiconductor laser and the like, a first lens 102, a second lens 103 whose focal length is f and a position detecting element 104 using PSD or CCD. The optical system in FIG. 7 is structured in order to fulfill the Scheimpflug condition. That is, the following equation (Equation 1) shows the relations of (1) the angle .theta. formed by the optical axis of the light source 101 and the optical axis of the second lens 103, (2) the angle .beta. formed by the principal plane of the second lens 103 and the detecting plane of the position detecting element 104, (3) the distance L from the intersection O of the optical axis of the light source 101 and the optical axis of the second lens 103 to the second lens 103, and (4) the distance d between the intersection of the optical axis of the light source 101 and the principal plane of the second lens 103 and the principal point of the second lens 103. EQU .beta.=tan.sup.- (f0/d), Equation 1
where f0=f.multidot.L/(L-f). In Equation 1, f indicates the focal length of the second lens 103. When the Scheimpflug condition is fulfilled, the optical beam reflected by the object to be measured through the first lens 102 after exiting from the light source 101 forms images on the position detecting element 104 by the second lens 103 in case where the object to be measured Ob is located not only at the intersection O of the optical axis of the light source 101 and the optical axis of the second lens 103 but also at the position O' deviated from the intersection O on the optical axis as shown by the broken line.
The principle of measuring distance by using this kind of range sensor is explained as follows. The part of the light reflected from the object to be measured Ob forms images at the point A on the position detecting element 104 by the second lens 103, when the object to be measured Ob is located at the position O. The part of the light reflected from the object to be measured Ob forms images at the point B on the position detecting element 104 as shown by the broken line in FIG. 7, when the object to be measured Ob has moved to the position O'. Thus, the position on the optical axis of the light source 101 is converted into the position of the image-formation on the position detecting element 104. Consequently, the shift, unevenness and the like of the object to be measured can be determined by measuring the shift of the position of the image-formation on the position detecting element 104.
However, the position detecting element mentioned above has the following problems. When PSD is used as a position detecting element and the position is detected by the sampling rate of more than 10 MHz, a resolution (normalized by the length of PSD) is on the order of 1/100, which is not enough. On the other hand, when CCD, which is for example a line sensor of 5000 pixels, is used as a position detecting element, the resolution (normalized by the length of CCD) reaches 1/5000. However, the scan rate of CCD is only on the order of several KHz and it is difficult to realize the high speed scan like at the scan rate of 10 MHz. Thus, it was difficult to detect a position at high speed (more than 10 MHz) and high precision (high resolution) using the conventional position detecting elements. As a result, it was difficult to have both the high speed performance of more than 10 MHz and the distance measurement of the high precision in the range sensor using this kind of position detecting elements.